Making a Sail-Shade-- Plane with curves
In addition to Duarte's answer, I'll  add another method. It will use the subdivision surface modifier as well, but another method to create the sharp corners


Add a plane. Subdivide it once. Add a solidify, bevel and subdivision surface modifier.

Since the sail would have a thickness anyway, the bevel modifier comes in handy. Otherwise just scale the value down untill it is no longer noticeable (but no 0). Use a high number of segments and a high angle. The angle has to separate the corners from the sides. The corner angles are going to be > 90 in your example, so I entered 80°.

You can now easily edit the shapes. Other shapes (not just quads) are equally possible as long as there is a distinction between side angle and corner angle.





You could also use NURBS surfaces. Add a surface, delete the standard additional points and close the endpoints, by checking Endpoint U and V.